1. Field of the Invention
The invention relates to a non-volatile semiconductor memory device, and in particular to a multilevel sense device for a flash memory.
2. Description of the Related Art
In a traditional semiconductor memory device, when a datum is written to a cell of an erasable programmable ROM (EPROM), charges are transmitted to a floating gate via a tunnel insulating film. At this time, if the cell is at a first state, such as a low logic level "0", the threshold voltage (V.sub.TH) thereof is set at a high logic level voltage. In contrast, if the charges on the floating gate are removed, the cell is charged into a second state, such as a high logic level "1", and the threshold voltage thereof is set at a low logic level voltage. When a digital datum stored in the cell is read, whether the digital datum is a logic level "1" or "0" can be determined/fetched by sensing the threshold voltage of the cell. A datum with one of the first state and the second state, can be stored in the traditional EPROPM by applying a threshold voltage lower than the high logic level or higher than the low logic level to the control gate. However, each time only an one-bit datum can be stored in a cell during programming. Therefore, the traditional EPROM has a disadvantage of small memory capacity.
Referring to FIG. 1, a traditional sense device for a flash memory, which can read 2-bit digital data, is shown. Typically, digital data stored in the flash memory are read out bit by bit. For example, when reading 2-bit digital data "10" which are stored in a flash cell 10, the first bit (high bit) "1" is transmitted to a comparator 12 to compare it with a reference value of a first reference array 14 so as to output the high bit "1". The second bit (low bit) "0" is transmitted to the comparator 16 to compare it with a reference value of a second reference array 18 so as to output the low bit "0". It is obvious that n-bit digital data require n reference arrays for comparisons. Therefore, it takes a great amount of time for data reading. In addition, the traditional method for data reading makes the entire layout areas of devices larger under the development of high memory capacity.
As described above, the traditional flash memory has no enough memory capacity owing to only a small number of bits stored in each cell thereof for access. Furthermore, it takes a great amount of time to read digital data bit by bit. Many reference arrays are needed for completing the purpose of reading, resulting in a difficulty for miniaturization.